1. Field of the Invention
The present invention provides improved spoke edge geometry for a non-pneumatic or hybrid tire that is less prone to fatigue when used. The present invention also provides a way to manufacture such geometry in a mold. In particular, the spoke edge geometry is provided with a reduced crass-section that reduces the bending stresses locally and allows a unique mold construction that changes the placement and orientation of potential flash and reduces other potential molding flaws when a liquid such as polyurethane is introduced into the cavity of the mold to form a spoke. This change results in a reduction in the possibility of a stress riser being found near the edge of the spoke, enhancing the durability of the tire.
2. Description of the Related Art
Non-pneumatic or structurally supported tires have been disclosed in the art. For example, U.S. Pat. No. 7,201,194, commonly owned by the applicant of the present invention, relates to a structurally supported resilient tire that supports a load without internal air pressure. The content of this patent is hereby incorporated by reference in its entirety. In an exemplary embodiment, this non-pneumatic tire includes an outer annular shear band and a plurality of web spokes that extend transversely across and radially inward from the annular band and are anchored in a wheel or hub. In certain exemplary embodiments, the annular shear band may further comprise a shear layer, at least a first membrane adhered to the radially inward extent of the shear layer and at least a second membrane adhered to the radially outward extent of the shear layer. In addition to the ability to operate without a required inflation pressure, the invention of U.S. Pat. No. 7,201,194 also provides advantages that include a more uniform ground contact pressure throughout the length of the contact area. Hence, this tire mimics the performance of a pneumatic tire.
FIG. 1 shows such a tire defining radial R and axial A directions. For reference, all the reference numerals in the 100's used herein refer to a previous tire, spoke and mold design while all reference numerals in the 200's used herein refer to a new and improved tire, spoke and mold design according to an embodiment of the present invention. The tire 100, 200 comprises a tread 102, 202 that is attached to the outward extent 104, 204 of the spokes 106, 206, which in turn, are connected to a hub or wheel 108, 208 at their inward extent 110, 210 by means known in the art. For the version of the tire 100, 200 shown, the spokes 106, 206 are formed by pouring a polyurethane liquid into a rotational mold where the liquid is then cured or hardened. It can also be seen that the spokes 108, 206 are grouped in pairs and that the individual spokes 106′, 106″, 206′, 206″ within each pair are consistently spaced from each other and that each pair is spaced consistently from the adjacent pair around the circumference of the tire. The spacing within each pair and the spacing between each adjacent pair do not need to be the same.
As described by the Abstract and col. 2, lines 28-41 of the '194 patent, the spokes 106, 206 support the tire 100, 200 in tension near the top of the tire 100, 200 and not in compression at the bottom of the tire 100. Instead, the spokes 106, 206 at the bottom of the tire near the contact patch, which is where the tread 102, 202 of the tire contacts the road, compress or buckle easily. This helps the tire to simulate the pneumatic support function of a pneumatic tire. As can be imagined, these spokes 106, 206 undergo a great deal of cyclic stress from tension to compression especially as the tire 100, 200 rotates at high speeds. This creates a risk of fatigue failure for the spokes. Consequently, the endurance of the spokes 106, 206 and the operability of the tire 100, 200 depend significantly on the accuracy of the geometry with which the spokes 106, 206 are made and the lack of any stress risers caused by manufacturing flaws.
Looking now at FIGS. 2A, 2B and 2C, front, side and sectional views respectively of a previous spoke design that was susceptible to molding flaws are shown. For the sake of clarity, the tread has been omitted. Focusing on FIG. 2C, the cross-sectional shape of spokes 106′, 106″ can be seen. The thickness of the spoke, T106, which is relatively consistent at 4 mm, and the edges 112′, 112″ of the spokes 106′, 106″ where flash 114 frequently occurs during the molding process are illustrated. The flash 114 is located near the edges 112′, 112″ of the spokes 106′, 106″ where radii 116 have been added to aid in stress reduction as the spokes 106′, 106″ cycle between tension and compression as the tire 100 rotates on a road surface under a vertical load. The reason why this flash occurs and why it is located as illustrated will be discussed more fully later. Since the cross section of the spokes 106′, 106″ is fairly straight and constant, the neutral axis or plane 118 about which each spoke 106′, 106″ flexes is essentially on the mid-plane of the spoke 106′, 106″ and the bending moment from a straight exterior surface 120 of the spoke 106′, 106″ to the neutral plane 118 remains fairly constant all the way to either edge of the spoke 106′, 106″.
In addition to the flash 114, the manner in which the mold that formed this geometry was built creates the possibility of mold mismatch from one side of the mold to the other which means that in addition to or sometimes instead of the presence of flash 114, the filleted edges 116 of the spokes 106 do not line up exactly with a straight exterior surface 120 of the spoke 106, creating a small ledge or corner near the edge of the spoke 106. This too can be undesirable for reasons that will be discussed below. A more complete explanation for this molding flaw will be discussed later.
Testing of this spoke design has revealed that any of these locations of flash 114 or mold mismatch create a stress riser as the spoke 106 cycles between tension and compression as the tire 100 rolls on a road surface. These manufacturing flaws then lead to crack initiation and propagation that can cause the spoke 106 to fail, undesirably impairing the operability of the tire 100. The location of these flaws is less than optimal because they are found near the edge 112 of the spokes 106 where they bend, creating high strains and stresses which cause cracks to initiate. Also, the orientation of the flash 114 is less than optimal since it is perpendicular or oblique to the neutral bending plane 118 of the spoke 106, which means that the flaw it creates is aligned with the direction in which the flash has a natural tendency to propagate cracks as the longest dimension of flash is the one that is bent, creating the highest moment and largest stress concentration in the flash. Put into other words, the flash is oriented in its most rigid configuration relative to the bending of the spokes making it more susceptible to cracking and this adds to the susceptibility of the spoke to fail 106.
Turning to FIG. 3, a general representation of how the mold 122 that made the previous spoke configuration was constructed is depicted. A first set of cores 124 that extend from a first mold half 126 and that Interarticulate with a second set of cores 128 that extend from a second mold half 130 form the majority of the surface area of the cavities 132, which are the negative image of the spokes that are formed. Each core has a 0.25° of draft on a side and this in conjunction with the interarticulation of the cores 124, 128 allows the spokes to maintain a constant thickness which helps maintain the strength of the spokes. It should be noted that these cores 124, 128 are actually arranged in a circular array in the mold 122 and that this figure shows their cross-sections projected onto a flat plane for ease of illustration. Also, common mold features such as venting for helping proper mold fill by allowing the escape of trapped gas and alignment features such as taper pins for facilitating mold alignment for the cores 124, 128 and mold halves 126, 130 have been omitted for the sake of clarity. Also, the cores are shown to be solid extensions of the mold halves 126, 130 but in actuality these are often separate inserts that are retained within the mold halves 126, 130 and that can be easily replaced should a core 124, 128 be damaged.
Looking more closely at the ends 133 of the cavities 132 that form the fillets found on the spokes, it can be seen that they are found adjacent to flat shut off surfaces 134 where the core 124, 128 extending from one mold half 126, 130 contacts or nearly contacts the other mold half 126, 130. As a result of this mold configuration, it is possible for a liquid such as polyurethane to seep into this space if a large enough gap is created due to machining tolerances, core deflection due to mold processing conditions, etc. This creates the undesired. flash that has been previously described near the edges of the spokes. Also, since the parting line is perpendicular to the direction of the extension of the cores 124, 128, the flash will be nearly orthogonal to the bending plane of the spokes, which is undesirable as explained above.
Looking now at FIG. 3A, which is an enlarged view of the radiused end portion 133 of the cavities 132, an example of mold mismatch is given. As shown, the core 128 extends undesirably into the cavity 132, creating a ledge or corner 138 that forms the complimentary shaped ledge or corner geometry in the spoke. In this case, either the location of the radiused end 133 is in the improper place due to manufacturing errors and/or tolerance stack ups, and/or the core is deflected, improperly manufactured, etc. so that the straight surface 138 of the core 128 is not tangent to the radiused end 133 of the cavity 132 but is shifted downward relative to the radiused end 133 of the cavity 132 as seen in FIG. 3A. Sometimes, this geometry is reversed and the core 128 is shifted upward relative to the radiused end 133 of the cavity 132 as seen in FIG. 3A. In either case, the ledge 138 that mold mismatch creates may also create a stress riser that is undesirably positioned and oriented since it is located on an outside surface near the edge of the spokes and is perpendicular to the natural bending plane of the spoke. So this too can initiate cracks that could cause the spoke to fail. Mold mismatch may occur in any, all or none of the cavities of the previous mold construction depending on a host of variables such as core deflection due to mold processing conditions, improper machining, and tolerance stack ups, etc.
Accordingly, there is a need for an improved spoke edge design and mold for creating this geometry that limits the creation and changes the orientation of molding flaws such as flash and mold mismatch near the edge of the spokes. Also, revised spoke edge geometry for reducing the strains and stresses found in this area would be helpful.